CERN Courier

Blurred vision helps spiders see better

People estimate distance using binocular vision and many animals do so by moving their heads round. However, jumping spiders, which need to be excellent judges of distances (as the name suggests, they jump!), use neither method. Remarkably, as Takashi Nagata of Osaka University in Japan and colleagues have recently shown, they make use of blurry vision.

Jumping spiders have two pairs of forward-facing eyes: the principal, more central eyes; and the anterior, lateral eyes. In studies of the Hasarius adansoni species, the researchers found that the principal eyes provide the depth perception and that these eyes have an unusual structure. Rather than having a single layer of photoreceptors, they have four, and it turns out that the first two are maximally sensitive to green light, the other two to ultraviolet. The effects of chromatic aberration mean that green images are focused clearly only on the first layer. The spiders jump accurately in green light but in other spectra they consistently underestimate distances. This appears (no pun intended) to be a genuinely new mechanism of depth perception.